In the manufacture of glass fibers, glass batch ingredients are melted in furnaces. The molten glass resulting from the melting of the glass batch mixtures is then fed into elongated forehearths or channels of various sizes and shapes. The forehearth forms an enclosure for the molten glass. The molten glass flowing in the forehearth is removed at many locations positioned along the length of the forehearth for subsequent processing. There are two major different forms of processing glass fibers.
The first major type of processing is for discontinuous glass fibers such as those used to manufacture insulation products and is described as a wool process. In the wool process, a ceramic or refractory block with a conical shaped cylindrical opening called a flow block is placed in an opening in the floor of a forehearth. This flow block allows the molten glass from the forehearth to flow in a uniform manner downward into a bushing block. The bushing block is a ceramic material which supports and surrounds a bushing having a cylindrically shaped opening. The bushing can be heated or cooled to provide a means to control the flow of the molten glass through the bushing. The bushing and bushing block have a flat upper face which is held against the lower face of the flow block. The molten glass passes through the bushing as a stream whose volume is controlled by the heating or cooling of the bushing. The molten glass stream flows into a spinner where it is centrifuged into discontinuous glass fibers.
The second major process for producing glass fibers produces continuous glass fibers such as are used for reinforcements for fiberglass reinforced products. This process is called a textile process. Again, a ceramic or refractory flow block is located in the opening of a forehearth. The flow block also has an aperture or cutout section sloping downwardly. This aperture may be circular, rectangular or be composed of a series of openings. The flow block permits the molten glass to flow uniformly from the forehearth to a fiber forming bushing beneath the flow block. The glass fiber producing bushing is a generally rectangular shaped container of precious metal, open at the top having a plurality of holes or orifices at the bottom to allow the molten glass to pass through the orifices to produce glass fibers. The fiber glass bushing itself is mounted, typically, in a steel or iron framework. The frame has within its confines a castable ceramic surrounding the bushing metal to electrically isolate the bushing from the frame material, since during operation the bushing is electrically heated to maintain a given, uniform temperature.
Regardless of which type of process is used to produce glass fibers, it is necessary periodically to replace the bushings used in the process. It is almost impossible because of the number of bushings attached to a given forehearth to drain the forehearth of molten glass to effectuate this bushing change. The current method of changing a bushing involves removing power from the bushing and spraying the bushing with water to cool the bushing. This solidifies the glass in the bushing and the bushing block. Cooling is continued until the glass in the flow block solidifies. The steel or iron framework holding the bushing and bushing block is then removed. The bushing is separated from the solidified glass by chiseling the bushing away from the glass. The solidified glass in the flow block region is then smoothed with chisels or air hammers to present a smooth flat surface. The new bushing, bushing block and framework or support is then mounted in position and attached. Cooling water flow is discontinued, electrical power is reapplied to the bushing and the solidified glass in the flow block is allowed to melt.
The cooling of the bushing and bushing block by directing water on them causes considerable thermal shock to the refractory of the forehearth. The cooling of this region of the forehearth to the extent that glass solidifies in the flow block has an adverse effect on the remainder of the molten glass flowing in the forehearth. The physical removal of the bushing and bushing block and the smoothing of the solidified glass in the flow block by mechanical means causes considerable physical shock to the forehearth refractory and its supports. Potentially, portions of the forehearth refractory can be broken away seriously affecting the mechanical strength and integrity of the forehearth.
As can be seen from the above discussion, a need exists to be able to change bushings in a glass fiber producing environment without the associated thermal and physical shock associated with the current method.